This invention relates to the use of chitosan to improve the flat crush resistance in the manufacture of corrugating medium. Specifically, the invention relates to the use of chitosan to improve the flat crush resistance of corrugating medium wherein the corrugating medium is in a fibrous suspension containing residual anionic degraded lignins remaining from the semichemical pulping processes.
In manufacturing corrugating medium the most widely used process is the neutral sulfite semichemical pulping process. This process is also the most popular for semichemical pulping generally.
The cooking liquor used in the neutral sulfite semichemical process is sodium sulfite. Indeed, the term "neutral sulfite" is derived from the fact that the sodium sulfite is used as cooking liquor at a pH of about 7.
The neutral sulfite semichemical (NSSC) process has advantages over other alkaline pulping processes. Some of these are that the neutral sulfite semichemical process produces pulps which make more rigid papers, the process produces high yields for a given degree of delignification, generally high strength, a wide applicability to different wood species, especially those little used in other processes, for example oak, gum, aspen and birch, and the hemicellulose content of the pulp manufactured by the NSSC process is similar to that of the original wood (because the xylose polymers are not hydrolyzed by the relatively neutral liquor.)
There is a trend in the pulping industry to use green liquor as the cooking liquor in place of sodium sulfite in the neutral sulfite semichemical process. The processes which use the green liquor have been termed the green liquor semichemical (GLSC) process and the modified kraft process. See, for example, PAPER TRADE JOURNAL, Oct. 14, 1974, page 25 and PULP AND PAPER, October, 1974, page 56, respectively. These articles are incorporated herein by reference. For the purposes of this invention, these processes, and any other process which uses spent green liquor, can be termed the green liquor process. The green liquor process has all the advantages of the neutral sulfite semichemical process, while being able to utilize spent green liquor directly as the cooking liquor for manufacturing corrugating medium. The green liquor process also produces high yield pulp. Finally, the green liquor process also has the advantage that less total chemical consumption should be needed to produce the same yield of corrugating medium pulp than other semichemical pulping processes.
The degree of separation of the semichemical pulp from the spent semichemical pulping liquor, varies from paper mill to paper mill but is usually not complete. With the trend toward closing-up of papermaking whitewater recirculation systems, these spent liquors can build up to high concentration levels. What has generally been lacking in these semichemical pulping processes is a composition of matter which can be used to increase the flat crush resistance at any effective level of spent green liquor in the manufacture of corrugating medium pulp. The utility of a composition of matter to improve the flat crush resistance will result in the following advantages to the corrugating medium: enable faster production rates of corrugating medium; and less corrugating medium will be necessary to achieve the same strength as corrugating medium without the added composition, i.e. the number of flutes per foot of board and the height of the flutes may be reduced because the strength of the corrugating medium fibers will be improved. Applications requiring double--board or triple--wallboard may be filled by single-lined board or regular corrugated board because of the improved flat crush resistance. Also, the thickness of the corrugating medium, which is usually 0.009 inches in caliper may be reduced to effect the same flat crush resistance of a corrugated board using 0.009 inch corrugating medium.
A known method for improving the flat crush resistance of corrugating medium is to impregnate it with molten sulfur. The method requires a post-treatment, where large amounts of sulfur are driven into the medium by pressure. The post-treatment is also relatively expensive and smelly to use. See, eg., Pulp and Paper, J. P. Casey, vol. III, p. 1920, Interscience, N. Y., 1961.
The use of chitosan that I have discovered as useful to improve the flat crush resistance of corrugating medium is superior to sulfur board. Because it is a cationic polymer, chitosan forms an interfiber bond with the corrugating medium fibers. That is, chitosan can be added during the pulping process and not as a surface additive. Therefore, chitosan will not interfere with the adhesion of the corrugating medium to the liner board. Much lower amounts are needed than for sulfur treatment. Finally, special adhesive formulas do not have to be used with the corrugating medium improved by chitosan. Also, a normal amount of adhesive is sufficient to obtain acceptable bonding. All of these properties are improvements over corrugating medium impregnated with molten sulfur to improve the flat crush resistance.
Because chitosan is a cationic polymer, it has an electrostatic attraction to the naturally occurring anionic properties of cellulosic fibers. For this reason, chitosan has been used to improve the dry strength of paper. See, for example, U.S. Pat. Nos. 3,709,780 and 2,047,218 which are incorporated herein by reference.
Other cationic polymers, see e.g. U.S. Pat. No. 3,875,097, also are dry strengthening agents. As a general statement, the degraded anionic, water-soluble lignins present in spent semichemical pulping liquor interact with these other cationic polymers and reduce their dry strengthening properties, making them ineffective in pulps of this type.